A previously demonstrated process for making such dense arrays of holes in glass is disclosed in U.S. Pat. No. 6,754,429. The disclosed method involves glass exposure with a femtosecond laser followed by acid etching. It relies on selective etching in which the laser-damaged glass etches at a significantly higher rate than the rest of the glass. Although technically sound, this approach utilizes expensive and maintenance-hungry femtosecond laser technology, and the laser exposure process is relatively slow.
According to US Patent Publication No. 20030150839, tapered (conical) holes 120-130 μm in diameter may be made by laser ablation followed by acid etching to remove surface defects and chips. The disclosed process requires an ion-exchange step before laser irradiation. Irradiation conditions beyond laser spot size and fluence are not disclosed.
US Patent Publication 20090013724 describes hole formation by laser irradiation and acid etching in glasses of various compositions. Lasers with wavelengths 355 nm and 266 nm were used. The recommended (numerical) beam aperture is NA<0.07 and the focus is disclosed as either within the glass or behind the back surface. Hole profile and placement accuracy are not specifically addressed.
What is needed is a relatively low-cost and reliable process for forming relatively small holes at relatively tight minimum pitch, with good positioning accuracy and reasonably small variation in diameter throughout the depth.
According to one aspect of the present disclosure, fabricating a high-density array of holes in glass, the method comprises providing a glass piece having a front surface, then irradiating the front surface of the glass piece with a UV laser beam focused to a focal point within +/−100 um of the front surface of the glass piece most desirably within +/−50 um of the front surface. The lens focusing the laser has a numerical aperture desirably in the range of from 0.1 to 0.4, more desirably in the range of from 0.1 to 0.15 for glass thickness between 0.3 mm and 0.63 mm, even more desirably in the range of from 0.12 to 0.13, so as to produce open holes extending into the glass piece 100 from the front surface 102 of the glass piece, the holes having an diameter the in range of from 5 to 15 μm, and an aspect ratio of at least 20:1. For thinner glass, in the range of from 0.1-0.3 mm, the numerical aperture is desirably from 0.25 to 0.4, more desirably from 0.25 to 0.3, and the beam is preferably focused to within +/−30 um of the front surface of the glass. The laser is desirable operated at a repetition rate of about 15 kHz or below, and generally of sufficient irradiation duration to form an open hole extending just up to a back surface of the glass piece. An array of holes thus produced may then be enlarged by etching. The front surface may be polished prior to etching, if desired.
Variations of the methods of the present disclosure are described in the text below and with reference to the figures, described in brief immediately below.